fix(sdk-coin-flrp): subtract minImportToCFee from ExportInP outputAmount

For P→C exports (ExportInP), the exported UTXO amount includes the
C-chain import fee that is added by the platform during transaction
building (amount = userAmount + importToCFee). Without adjusting the
outputAmount, the confirmation UI reads the gross UTXO amount as the
displayed send amount. When the platform additionally applies its own
import-fee subtraction from the same gross value, the resulting
spendAmount can appear as 0 or be misinterpreted, causing the export
confirmation UI to display "- TFLR" instead of the correct amount.

Fix: in Flrp.explainTransaction, when the transaction is a P-chain
export (ExportInP, !isTransactionForCChain), subtract minImportToCFee
from outputAmount and each output's amount. This yields the minimum
expected net C-chain receipt, aligning the confirmation display with
the amount the user will actually receive after ImportInC, consistent
with how ExportInC subtracts minImportToPFee (CECHO-1450).

Add minImportToCFee to the FlareNetwork interface and set it to
2850000 nFLR (250 nFLR/gas × ~11400 gas, the minimum EVM import fee).
Add test coverage verifying ExportInP outputAmount is adjusted.

Ticket: CECHO-1518
Session-Id: 6f2ea6bd-01d6-4a82-a82f-0af0974e5165
Task-Id: 65e37cf1-5c7c-4605-8126-070b8d699a02

Author: ArunBala-Bitgo

modules/sdk-coin-flrp/src/flrp.ts

@@ -451,6 +451,24 @@ export class Flrp extends BaseCoin {
         };
       }
 
+      // For a P→C export (ExportInP), the exported UTXO amount includes the C-chain
+      // import fee that will be deducted when ImportInC is confirmed. Subtract the
+      // minimum import-to-C fee so the displayed amount reflects the expected net
+      // C-chain receipt, consistent with how ExportInC subtracts minImportToPFee.
+      if (!tx.isTransactionForCChain && explanation.type === TransactionType.Export) {
+        const minImportToCFee = BigInt((this._staticsCoin.network as FlareNetwork).minImportToCFee);
+        const adjustedOutputs = explanation.outputs.map((o) => ({
+          ...o,
+          amount: (BigInt(o.amount) - minImportToCFee).toString(),
+        }));
+        const adjustedOutputAmount = (BigInt(explanation.outputAmount) - minImportToCFee).toString();
+        return {
+          ...explanation,
+          outputs: adjustedOutputs,
+          outputAmount: adjustedOutputAmount,
+        };
+      }
+
       return explanation;
     } catch (e) {
       throw new Error(`Invalid transaction: ${e.message}`);

modules/sdk-coin-flrp/test/unit/flrp.ts

@@ -313,6 +313,30 @@ describe('Flrp test cases', function () {
       signedExplain.outputs[0].amount.should.equal(expectedAdjustedAmount);
     });
 
+    it('should subtract minImportToCFee from ExportInP outputAmount to show expected net C-chain receipt', async () => {
+      // The ExportInP UTXO amount is the gross amount going to C-chain, which includes
+      // the C-chain import fee that will be deducted when ImportInC is confirmed. We
+      // subtract minImportToCFee to show the minimum net amount the user will receive
+      // on C-chain, preventing the confirmation UI from showing a higher-than-actual
+      // amount and causing the display to appear as "- TFLR" instead of the correct amount.
+      const minImportToCFee = BigInt('2850000'); // from FlarePTestnet.minImportToCFee
+      const grossOutputAmount = BigInt(EXPORT_IN_P.amount); // 55000000 (includes import fee)
+      const expectedAdjustedAmount = (grossOutputAmount - minImportToCFee).toString();
+
+      // Should work for both half-signed and fully-signed ExportInP hex
+      const halfSignedExplain = await basecoin.explainTransaction({
+        halfSigned: { txHex: EXPORT_IN_P.halfSigntxHex },
+      });
+      halfSignedExplain.outputAmount.should.equal(expectedAdjustedAmount);
+      halfSignedExplain.outputs.should.be.an.Array();
+      halfSignedExplain.outputs.length.should.equal(1);
+      halfSignedExplain.outputs[0].amount.should.equal(expectedAdjustedAmount);
+
+      const signedExplain = await basecoin.explainTransaction({ txHex: EXPORT_IN_P.fullSigntxHex });
+      signedExplain.outputAmount.should.equal(expectedAdjustedAmount);
+      signedExplain.outputs[0].amount.should.equal(expectedAdjustedAmount);
+    });
+
     it('should fail when transaction hex is not provided', async () => {
       await basecoin.explainTransaction({}).should.be.rejectedWith('missing transaction hex');
     });

modules/statics/src/networks.ts

@@ -18,6 +18,7 @@ export interface FlareNetwork extends BaseNetwork {
   vm?: string;
   txFee: string;
   minImportToPFee: string;
+  minImportToCFee: string;
   maxImportFee: string;
   createSubnetTx?: string;
   createChainTx?: string;
@@ -2318,7 +2319,8 @@ export class FlareP extends Mainnet implements FlareNetwork {
   hrp = 'flare';
   alias = 'P';
   vm = 'platformvm';
-  minImportToPFee = '1261000'; // 0.1261 FLR
+  minImportToPFee = '1261000'; // minimum PVM import-to-P fee in nFLR
+  minImportToCFee = '2850000'; // minimum EVM import-to-C fee in nFLR (250 nFLR/gas * ~11400 gas)
   txFee = '200000'; // FLR P-chain import requires higher fee than base txFee
   baseTxFee = '1000000';
   maxImportFee = '10000000'; // defaults
@@ -2354,7 +2356,8 @@ export class FlarePTestnet extends Testnet implements FlareNetwork {
   alias = 'P';
   assetId = 'fxMAKpBQQpFedrUhWMsDYfCUJxdUw4mneTczKBzNg3rc2JUub';
   vm = 'platformvm';
-  minImportToPFee = '1261000'; // 0.1261 FLR
+  minImportToPFee = '1261000'; // minimum PVM import-to-P fee in nFLR
+  minImportToCFee = '2850000'; // minimum EVM import-to-C fee in nFLR (250 nFLR/gas * ~11400 gas)
   txFee = '200000'; // FLR P-chain import requires higher fee than base txFee
   baseTxFee = '1000000';
   maxImportFee = '10000000'; // defaults